Recently, with an increasing integration degree of a semiconductor device, the dimensions of individual elements have become finer, and the widths of wiring and gate constituting each element have also become finer.
EUV (Extreme Ultraviolet) lithography and nanoimprint lithography (NIL) have attracted attention as technologies for forming fine patterns on a semiconductor wafer. Since the EUV lithography uses extreme ultraviolet light as a light source to transfer patterns of EUV mask onto the wafer, it is possible to form finer patterns on the wafer than a conventional exposure apparatus using ArF light. In the nanoimprint lithography, a fine pattern is formed in a resist by pressuring a template having a nanometer-scale fine structure to the resist on the wafer. In both the EUV lithography and the nanoimprint lithography, a pattern formed in the EUV mask and the template being an original plate is finer when compared with conventional ArF lithography. Thus, high inspection accuracy is required for the inspection thereof.
An exposure apparatus called a stepper or a scanner is used in the transfer process in the EUV lithography. In the exposure apparatus, light is used as a transfer light source, and a circuit pattern on the mask is projected onto the wafer while reduced from about one-fourth to about one-fifth size. Accordingly, the dimension of a circuit pattern to be formed in the mask is about four times to five times as large as a dimension of a circuit pattern on the wafer. On the other hand, in a template used in the nanoimprint lithography, a circuit pattern, having the same dimension as a circuit, is formed by digging a printing plate down to a predetermined depth. In a contemporary semiconductor device, a line width of a pattern or a width of a space between patterns might be ten nanometers to several tens of nanometers, and a depth of a dugout portion might be several tens of nanometers to one hundred nanometers.
Because the pattern of the template has the dimension identical to the dimension of the circuit, the defect existing in the template has a larger influence on the pattern imprinted to the wafer compared with the pattern of the mask. Because the template is used in the imprinting process a plurality of times, the defect is wholly imprinted to the wafer together with the pattern. Accordingly, in inspecting the pattern of the template, higher accuracy is required compared with the inspection of the pattern of the mask. For example, JP 2012-26977 A discloses an inspection apparatus for detecting the defect of the template.
The pattern formed in the template cannot be resolved in the case where the pattern is finer than a wavelength of a light source in an inspection apparatus. Generally the limitation of the dimension of the pattern that can be resolved, is known as a Rayleigh resolution limit. Nowadays, as microfabrication of the pattern of the circuit is progressed, it is possible that the dimension of a pattern becomes finer than the resolution limit of an optical system in an inspection apparatus.
Assuming that NA is a numerical aperture of an optical system in an inspection apparatus and that λ is a wavelength of the light source, the resolution limit of the optical system is expressed by the formula (1). The numerical aperture NA is usually in the range of approximately 0.7 to approximately 0.8. Further, k1 is a coefficient depending on an image forming condition, and is in the range of approximately 0.5 to approximately 1.R=k1(λ/NA)  (1)
In a contemporary semiconductor device manufacturing process, in inspecting the mask used in a reduced projection exposure of the circuit pattern to the wafer, the mask is irradiated with continuous light, having a wavelength of approximately 200 nm, close to the wavelength of the light source of the exposure apparatus. The light transmitted through or reflected by the mask is received by a sensor through a proper magnification optical system to obtain an electrical signal constituting an optical image of the mask. The dimension of the pattern formed in the mask is approximately four times as large as the line width (several tens of nanometers) of the pattern to be formed on the wafer, namely, approximately one hundred nanometers to several hundred nanometers.
In the formula (1), when the wavelength of the light source is set to 200 nm, and when the numerical aperture is set to 0.7, a formula (2) is obtained.R=0.5×(200/0.7)=143 (nm)  (2)
According to the formula (2), the resolution limit size in this case is 143 nm. That is, when the patterns of the mask come closer than 143 nm to each other, an electrical signal of a brightness amplitude corresponding to the pattern is not obtained by the sensor. This is similar for a pattern of a template. Because the pattern of the template has the same dimension as the circuit to be formed on the wafer, the pattern of the template cannot be resolved in principle. The shapes of some of the non-repetitive, slightly thick patterns such as a lead wire or a gate wire can occasionally be distinguished.
Instead of the inspection optical system provided with the above-mentioned light source, a method for acquiring a pattern by applying an electron beam or an atomic force is conceivable as a method for resolving the fine pattern to identify the defect. However, for the inspection in which the electron beam or the atomic force is used, there is a problem in that the inspection is not suitable for mass production of semiconductor device because of low throughput.
In the template in which a repetitive pattern finer than the resolution limit of the optical system in the inspection apparatus is formed, when a reflection optical image of the template is acquired, the optical image (electrical signal image) has brightness corresponding to a film quality of the template at a location where the pattern is not arranged. For example, the optical image becomes an even brightness close to a white level determined by calibration. At a location where the pattern is arranged, the optical image has the brightness different from that at the location where the pattern is not arranged, for example, the optical image is observed as an even gray image, that is, between the white level and a black level in the brightness.
On the other hand, when the defect exists at the location where a predetermined pattern is periodically formed, the periodicity of the pattern is disturbed, and the optical image becomes an image in which the brightness is changed according to a degree of the defect in the even gray image. For example, the brightness change is observed as an isolated white or black point.
The defect can be detected in the pattern finer than the resolution limit of the optical system by detecting the brightness change caused by the disturbance of the periodicity. Specifically, in the identical template, the defect is detected by using a die-to-die comparison method in which optical images of a plurality of dies are compared to each other or a cell comparison method in which optical images in the areas where the identical pattern is formed are compared to each other. For example, the two dies, which both appear to be the even gray image when the patterns have no defects, are compared to each other to determine that the image having the brightness change caused by the disturbance of the periodicity has the defect.
When the optical image is captured while a focal position between the pattern and the optical system is changed for the repetitive pattern finer than the resolution limit of the optical system, the brightness change, namely, a variation in gradation value is observed in each optical image. The variation in gradation value depends on the focal position. The focal position where the variation becomes the maximum is the position where the contrast of the optical image is maximized, namely, a focusing position. However, it is well known that a signal-to-noise (S/N) ratio of the defect inspection is occasionally improved when the defect is inspected while a given distance (focus offset) is intentionally provided with respect to the focusing position. Therefore, the focusing position where the contrast of the optical image becomes the maximum is obtained, and the inspection is performed using the position where the focusing position is corrected by the focus offset as the optimum focal position.
The focus offset also has an optimum value, and the optimum value depends on a type, a shape, and a dimension of the defect.
For example, it is considered that a line-and-space pattern is regularly arrayed with given periodicity. Assuming that the broken pattern defect is generated in the line-and-space pattern by disconnection, the defect is seen as a white bright spot in the even gray image when the defect is observed with the focus offset. At this point, when the focus offset is changed, the defect is seen as a black spot in the even gray image. In intermediate focus offset, amplitude of a defect signal cannot be obtained by an image sensor, and therefore the defect cannot be observed.
For example, when the defect, in which the adjacent line patterns are partially connected to each other to form a pattern bridge defect, exists in the line-and-space pattern, the pattern bridge defect caused by the short-circuit is seen as a black-and-white inversion of the broken pattern defect caused by the disconnection. That is, when the focus offset seen as the white bright spot in the broken pattern defect caused by the disconnection is applied to the pattern bridge defect caused by the short-circuit, the pattern bridge defect caused by the short-circuit is seen as the black spot while the black-and-white inversion of the broken pattern defect caused by the disconnection is generated. In the focus offset seen as the black spot in the broken pattern defect caused by the disconnection, the pattern bridge defect caused by the short-circuit is seen as the white bright spot.
In the above example, when the shape or dimension of the pattern bridge defect or broken pattern defect varies, the brightness of the defect, namely, the brightness of the white or black spot changes, or the focus offset in which the brightness becomes the maximum changes.
Therefore, in inspecting the template, the defect is detected by performing a preliminary inspection, the focus offset is adjusted using the defect, the optimum focus offset is found in order to detect the defect, and the inspection is performed. However, the focus offset cannot be adjusted when the defect is not detected in the preliminary inspection. For this reason, when the defect is not detected in the subsequent inspection, it is not distinguished whether the defect is not detected due to the actual absence of the defect or the improper focus offset, which results in a problem in that inspection quality cannot be guaranteed.
The present invention has been devised to solve the problem described above. An object of the present invention is to provide an inspection method for properly adjusting the focus offset to be able to accurately detect the defect of the pattern finer than the resolution limit of the optical system in the inspection apparatus.
Another object of the present invention is to provide an inspection method that can accurately detect the existence of a defect of a pattern finer than a resolution limit of an optical system of an inspection apparatus, by appropriately adjusting a focus offset, and reducing the influence of noise.
Further, another object of the present invention is to provide a template that can accurately detect the existence of a defect of a pattern finer than the resolution limit of an optical system of the inspection apparatus by accurately adjusting the focus offset.
Other advantages and challenges of the present invention are apparent from the following description.